1. Field of the Invention
The present invention relates generally to rotating drilling head systems in which an elastomer seals around and grips a rotating drill pipe during drilling operations.
2. Description of the Related Art
Oil and gas wells are typically drilled by use of a rotating drill pipe with a drill bit at the lower end. Drilling fluids are pumped down the drill pipe and out the drill bit. The drilling fluid returns to the surface, along with cuttings, through the annulus around the drill pipe. In many cases, the pressure at the upper end of the drill pipe annulus is atmospheric. The weight of the drilling fluid is controlled to provide a hydrostatic pressure at the earth formations that is greater than the formation pressure to prevent blowouts.
In some cases, however, it is advantageous to isolate the pressure at the upper end of the column from atmospheric pressure. For example, in highly deviated well, a lightweight drilling fluid may be used that is not heavy enough to prevent upward flow in the well due to formation pressure. A drilling head at the upper end of the well controls the pressure. Drilling head systems use an elastomeric element to seal the drilling head against the rotating drill pipe during drilling operations. In some rotating drilling head systems, the seal is formed by the natural resiliency of the elastomeric element against the drill pipe while others use hydraulic pressure to deform the seal element. In U.S. Pat. No. 6,016,880, hydraulic pressure is applied to a bladder that surrounds an elastomeric gripper element that is located above an elastomeric primary seal. The bladder forces the gripper inward to grip the drill pipe to cause the gripper and primary seal to rotate with the drill pipe. The gripper also serves as a secondary seal in the event of leakage of the primary seal. Furthermore, the gripper has to continue gripping and sealing around the drill pipe as it moves downward while drilling. The tool joints at the end of each drill pipe are larger in diameter than the drill pipe and must pass through the gripper while it continues to seal and grip the drill pipe.
While the system of the '880 patent is workable, improvements in the gripper are desirable. As the bladder forces the gripper element inward, the gripper deforms, but does not compress. The deformation results in high stress and strain.